package link_list;

/**
 * @author swing
 * @description
 * @create 2022 - 11 - 19 18:23
 */
public class HasLoop {
    public static void main(String[] args) throws Exception {
        Node<String> first = new Node<String>("aa", null);
        Node<String> second = new Node<String>("bb", null);
        Node<String> third = new Node<String>("cc", null);
        Node<String> fourth = new Node<String>("dd", null);
        Node<String> fifth = new Node<String>("ee", null);
        Node<String> six = new Node<String>("ff", null);
        Node<String> seven = new Node<String>("gg", null);
//完成结点之间的指向
        first.next = second;
        second.next = third;
        third.next = fourth;
        fourth.next = fifth;
        fifth.next = six;
        six.next = seven;
//产生环
        seven.next = third;
//判断链表是否有环
        boolean circle = isCircle(first);
        System.out.println("first链表中是否有环：" + circle);

        //查找环的入口结点
        Node<String> entrance = getEntrance(first);
        System.out.println("first链表中环的入口结点元素为：" + entrance.item);
    }


    /**
     * 判断链表中是否有环
     *
     * @param first 链表首结点
     * @return ture为有环，false为无环
     */
    public static boolean isCircle(Node<String> first) {
        Node<String> slow = first;
        Node<String> fast = first;
        //当快指针没有下个节点时停止遍历
        while (fast != null && fast.next != null) {
            fast = fast.next.next;
            slow = slow.next;
            if (fast.item.equals(slow.item)) {
                return true;
            }
        }
        return false;
    }

    public static Node getEntrance(Node<String> first) {
        Node<String> slow = first;
        Node<String> fast = first;
        Node<String> temp = null;
        while(fast!=null && fast.next!=null){
            fast = fast.next.next;
            slow=slow.next;
            if (fast.equals(slow)){
                temp = first;
                continue;
            }
            if (temp!=null){
                temp=temp.next;
                if (temp.equals(slow)){
                    return temp;
                }
            }
        }
        return null;
    }

    //结点类
    private static class Node<T> {
        //存储数据
        T item;
//下一个结点

        Node next;

        public Node(T item, Node next) {
            this.item = item;
            this.next = next;
        }

    }
}
